Elevators are presently provided with a plurality of braking devices which are designed for use in normal operation of the elevator, as for example to hold the cab in place when it stops at a landing and which are designed for use in emergency situations such as stopping the cab and/or counterweight from plunging into the hoistway pit.
Elevator safeties are typically devices mounted on the car frame or counterweight assembly which are tripped by sensed overspeed of a governor cable connected to the cab or counterweight. Once the safeties are tripped, they will typically grab the guide rails in the hoistway to stop the cab or counterweight. The elevator safeties described above are not operated in an instance where the cab is moving out of control in the upward direction in the hoistway.
Concerns as to passenger safety in an elevator cab moving out of control in the upward direction in a hoistway have prompted safety code revisions in North America which mandate that elevator systems include provisions for safely stopping a runaway cab in the upward direction. These revisions have prompted research and investigation into braking systems which can provide the required function. U.S. Pat. No. 4,977,982, granted Dec. 18, 1990 to L. Bialy, et al., discloses an elevator sheave brake safety which acts directly on the drive sheave to jam the latter in the case of uncontrolled upward movement of the elevator cab, whereby the cab is stopped in the hoistway. The brake safety can also stop movement of the cab away from a landing in the event that the cab doors are open. Both of these features are desirable safety features The brake safety disclosed in this patent utilizes a pair of wedge blocks which flank the sheave and are spring-biased toward the sheave. When the safety is tripped by undesirable cab movement, the wedges will move against and jam the sheave so that it stops rotating, thereby stopping the cab. The wedging can stop the cab in both the up and down directions. U.S. Pat. No. 5,007,505, granted Apr. 16, 1991 to R. Lindegger, discloses an elevator traction sheave brake which includes a vertically reciprocating spring-biased friction plate beneath the sheave. The friction plate is normally held away from the sheave, but during undesirable cab movement, it will move upwardly to jam rotational movement of the sheave. As before, this brake can stop upward or downward movement of the cab.
Both of the aforesaid brake assemblies operate directly on the drive sheave or a part attached thereto to jam the sheave at its circumference and can thus damage the sheave or create a wedged engagement with the sheave that is very difficult to release. Each of these brakes also results in minimal surface contact between the sheave and the brakes whereby forces exerted on the sheave are quite concentrated and whereby the cab will be jolted to a stop when the brakes trip.
There are several problems relating to the prior art elevator up direction safeties which remain to be solved. One problem relates to the development of a brake which can be easily retrofitted onto an existing elevator system in the field The ability of the brake to be readily released, to be reused without refurbishing, and to operate with a relatively low power supply are also highly desirable.